In conventional telephony, a basic channel format is the 64 kbit/s channel, in which a single speech connection is transmitted. The 64 kbit/s channel transmits 8000 samples per seconds, each sample having 8 bits. Typically, a plurality of 64 kbit/s channels are transmitted in a single transmission line to form 1544 kbit/s, 2048 kbit/, and higher transmission rates. In a 2048 kbit/s transmission link, a 256 bit i.e. 32 byte frame is transmitted 8000 times per second. The 32 groups of 8 bits, i.e. bytes of the frame are referred to as time slots. The terms PCM transmission line and trunk line are commonly used to refer to a communication link transmitting a plurality of 64 kbit/s channels. Therefore, to identify a certain speech channel transmitted within a group of trunk lines, one needs to indicate the time slot number of the channel and a trunk line identifier. The term trunk line is sometimes used also to denote a basic 1544 kbit/s or 2048 kbit/s unit of transmission even in a case, when a plurality of such transmission units are transmitted in a single physical transmission medium i.e. in a single physical transmission line. Although the acronym PCM strictly considered denotes pulse code modulation, which is typically used in trunk lines, the term PCM transmission line is generally used by a person skilled in the art and specifically in this specification to refer to aforementioned logical group of channels or a group of groups of channels and not to a specific modulation method.
Further, packet based transmission networks are presently in widespread use, a prominent example being the Internet. A common packet transmission protocol is the Internet Protocol (IP). The IP protocol version 4 is described in detail in the specification RFC 791. The next version of the IP protocol, known as IPv6, is described in the specification RFC 1883.
The increasing importance and use of telecommunication drives toward inter-connection of different types of networks. For example, the Internet is already used for transmitting voice using so called internet telephony. As the data transmission capacity of the Internet increases, the use of Internet as a replacement of conventional telephones will become common. Some telephone operators already provide long distance calls via the Internet with a reduced rate.
Publication WO 9914929 discloses method and apparatus for placing long distance telephone calls via a packet data network and local telephone calls via local telephone network, the user of the telephone set plainly dialling the desired destination number. The routing of the call via either packet data network or telephone network is based on the destination number.
Publication WO 9904540 discloses a system for establishing communication between two endpoints connected to each other via two communication media, for example Ethernet and ATM (Asynchronous Transfer Mode), operating in accordance with different communication protocols. A connection controller selects a connection path connecting the two endpoints and configures the endpoints and an interface between the two communication media to establish a connection before any data is sent using the connection path.
Publication WO 9716007 discloses a telecommunication system, where it is possible to transmit calls between terminals using a packet switched network or a packet switched network and telephone network. There is a gateway connecting a packet switched network to telephone network. A first connection is typically set up between a first terminal and the gateway via, for example, the packet switched network and a second connection is set up between a second terminal and the gateway via the telephone network.
FIG. 1 illustrates one possible arrangement for transmission of speech using an IP network together with a telephone network. FIG. 1 shows switching elements 10a, 10b, 10c of the telephone network being connected to each other with PCM communication links 12. Further, FIG. 1 shows an IP network 40, and routers 30 connected to the IP network. Network elements such as IP gateways 20 connect the switching elements 10a, 10b, 10c to the routers 30. Without using the IP network, speech data from a first party PARTY A of a connection to a second party PARTY B is transferred using the PCM connections 12. The call may pass through a large number of switching elements, depending on the geographical distance spanned by the call. When the IP network 40, such as the Internet network is used, a switching element directs the speech data to an IP gateway instead of another switching element, and the IP gateway sends the speech data through the IP network to distant IP gateway connected to a distant switch element. The distant IP gateway converts the received IP packets to a speech data stream and forwards the data stream to the distant switch element for further processing. One or more legs of a call may be routed in such a way through an IP network.
One example of the proceeding of call setup according to prior art is illustrated in FIG. 2. FIG. 2 shows the local telephone exchange 5 of a calling party PARTY A and switching elements 10a, 10b, 10c of the telephone network. First, the calling party initiates the call, whereby the local telephone exchange sends 100 an ISUP SETUP message to the next switching element 10a. The switching element 10a performs digit analysis 105 to determine, where the call should be directed. In this example, the digit analysis reveals, that the next switching element SW B 10b is an IP capable switch. As a consequence, switching element SW A 10a sends an ISUP SETUP message 110 to switching element SW B 10b. The switching element SW A 10a may include in the message an indication that it wishes to set up an IP connection, for example as a parameter specifying an IP address corresponding to the IP gateway of switch element SW A. The switching element SW B 10b responds by sending 115 a message such as a CHANNEL INFO message specifying an IP address corresponding to the IP gateway connected to switch element SW B. After receiving the IP address, switching element SW A may start sending 120 speech data to switching element B via the IP gateways and the IP network. After sending the CHANNEL INFO message, the switching element SW B performs digit analysis 125 to determine, where it should direct the call. In this example, the digit analysis reveals that the next switching element is switching element SW C 10c, and that SW C is IP capable. As a consequence, switching element SW B 10b sends an ISUP SETUP message 130 to switching element SW C 10c. The switching element SW B 10b may include in the message an indication that it wishes to set up an IP connection, for example as a parameter specifying an IP address corresponding to the IP gateway of switch element SW B. The switching element SW C 10c responds by sending 135 a message such as a CHANNEL INFO message specifying an IP address corresponding to the IP gateway connected to switch element SW C. After receiving the IP address, switching element SW B may start sending 140 speech data to switching element C via the IP gateways and the IP network.
The resulting situation is not optimal regarding the efficiency of data transfer: switching element SW B receives a speech data stream from the IP network, and returns the speech data stream back to the network for forwarding to switching element SW C.
The same situation may occur also in a cellular telecommunication network, in which case the switching elements 10a, 10b, 10c are switching elements of a cellular telecommunications network, such as mobile services switching centers (MSC) of a GSM (Global System for Mobile communications) or a UMTS (Universal Mobile Telecommunication System) network. In a cellular telecommunication network such a situation may also occur as a result of a inter MSC handover, for example when a mobile station (MS) under control of MSC SW B moves to an area under control of MSC SW C, while having a connection to a mobile station under control of MSC SW A.